Rotational end mills have long been utilized for various cutting duties. Conventionally, these end-mills are constructed with different types of hardened steel as well as tungsten carbide, and are often given additional structural features such as a corner radius at the cutting ends, tapered cutting ends, ball shaped cutting ends, uneven cutting edges for rough milling operations including serrations and still other edge contours. Likewise, these end-mills can be provided for longer wear with wear-resistant PVD and CVD coatings including amorphous diamond and various nitride compositions.
A common problem encountered in the use of end mills is “chatter”. When cutting ferrous and non-ferrous materials, especially at aggressive cutting feed rates, harmonics can generate regenerative vibration whereby the rotating end mill's frequency of vibration self-excites. The self-exciting chatter is usually accompanied by a loud and excessive noise while machining. One cause of this chatter is when the peripheral cutting edges formed along the helix are spaced at equal distances about the end mill such that the time between the cutting edges hitting the material being cut is the same (or even worse, in a concave radial cut when more than one cutting edge hits the material being cut at the same time and at the same intervals). Excessive chatter can result in a poor surface finish, rework or scrap of the work product. Chatter can damage the cutting edge of an end mill and limit its useful life, thereby increasing costs for the milling operation and generating less precise machined parts than may be desired or required for a particular final function. Excessive chatter can also cause premature wear to the actual milling machine and its components.
In order to combat the harmonics, variable helix end mills and variable pitch end mills have been developed. A variable helix end mill is generally an end mill having helical flutes in which the circumferential distance between the peripheral cutting edges varies in an axial direction along the end mill. The circumferential distance is also sometimes described as an angle between adjacent peripheral cutting edges known as the index angle or pitch. One type of variable helix end mill is when adjacent helical flutes have different helix angles. Another type is when the helical flutes have different variable helix angles (i.e. the helix angle of one flute is 40 degrees at the leading end of the flute and 35 degrees on the trailing end of the flute). The other type of end mill discussed is the variable pitch end mills. One type of variable pitch end mill is when all helical flutes have the same helix angle with the flute indexing altered from the typical 90 degree spacing. Unlike the variable helix end mills, the circumferential distance between adjacent peripheral cutting edges of a variable pitch end mill typically is constant in the axial direction of the end mill.
One of the most commercially successful variable helix end mills is the Z-Carb® end mill manufactured under U.S. Pat. No. 4,963,059, and owned by the Applicant. The U.S. Pat. No. 4,963,059 patent disclosed an end mill having a plurality of paired helical flutes forming an even number of helical peripheral cutting edges equally spaced circumferentially in one plane wherein the peripheral cutting edges are formed as a plurality of pairs of diametrically opposite cutting edges having the same helix angle and thereby being symmetrical with respect to the axis of the body. While the Z-Carb® end mill is resistant to chatter and provides a good surface finish, the technology is over 20 years old and it is believed that there is still room for improvement.
Many manufacturers of end mills have attempted to employ different strategies for reducing harmonics. One such attempt is described in US Published Patent Application US2004/0120777, which teaches an end mill having a plurality of flutes wherein each feature of the flute is unsymmetrical with each other flute including location of the flute about the tool (index angle), helix angle, radial rake angle, and radial relief angle. It would seem to follow that a tool having everything different would be the best performer in terms of the reduction of chatter, however, testing of these tools have shown a decrease in performance in comparison to other leading end mills. An end mill having all different features can have stability problems that may be even worse than the problems with chatter. Another problem with such tools is that the production and resharpening of the tool is difficult because all of the features of the end mill are different.
Another attempt to reduce harmonics and increase performance is taught in U.S. Pat. No. 6,997,651, entitled End Mill Having Different Axial Rake Angles and Different Radial Rake Angles. This prior art end mill has a plurality of flutes all having the same helix angle and being equally spaced about the circumference of the tool (same index angle), but having at least two different radial rake angles and at least two different axial rake angles. Like the other prior art end mill discuss above, the testing of this tools has shown a decrease in performance in comparison to other leading end mills, including in the reduction of harmonics. The performance of this end mill will be discussed in greater detail below.
Many other attempts have been made in the prior art to improve the performance of end mills with regards to chatter. The reduction of harmonics is not accomplished by making all features different in a random manner as this may have serious adverse consequences with the performance of the tool. Accordingly, there remains room for improvement in the prior art to reduce chatter without sacrificing stability of the tool.